This invention relates to chemical-mechanical planarization of semiconductor wafers, and more particularly to slurry filtration systems used in such machines.
Chemical Mechanical Planarization (CMP) is the preferred technique for globally planarizing semiconductor wafers at high levels of integration. In CMP, the semiconductor wafer is generally mounted in a wafer carrier disposed above a polishing pad that is attached to a rotatable platen. The exposed surface of the wafer is then pressed against the polishing pad with a prescribed down force, and the polishing pad and/or the wafer are then independently rotated while the wafer carrier is translated across the pad surface. While the semiconductor wafer is moved across the polishing pad, a polishing slurry is distributed across the surface of the pad to facilitate planarization of the wafer. The slurry is generally comprised of a combination of chemical etchants and very highly abrasive particles in a liquid suspension to simultaneously etch and abrade the wafer surface as it moves across the pad. Polishing slurry compositions commonly used in wafer planarization are generally comprised of abrasive compounds such as colloidal silicon dioxide or a dispersoid of alumina with particle sizes in the 0.01-0.3 micron range. Suitable chemical agents for etching the wafer are generally chemical compounds such as potassium hydroxide or ammonium hydroxide.
A significant problem encountered in CMP is surface damage to the wafer due to relatively large abrasive particles that scratch the surface of the wafer. This problem is partiaily addressed during the slurry manufacturing process, since the abrasive particles that comprise the slurry are sized so that abrasive particles of sufficient size to cause wafer scratching are eliminated. In a typical polishing slurry, however, abrasive particles that are unacceptably large may still be encountered, since the sizing procedure may not exclude all of the abrasive particles of unacceptable size. These abrasive particles, commonly referred to as xe2x80x9ctailsxe2x80x9d, generally exist in slurry formulations in proportion to the cost of the formulation because a reduction in the number of tail particles requires that the abrasive particle sizing be more rigidly controlled when the slurry is formulated.
The occurrence of surface scratching particles also results from abrasive particles that combine, or agglomerate, in the slurry to form particles that have an effective size significantly outside the range of acceptability. Thus, surface-scratching agglomerations may form even where the population of tail particles is very rigidly controlled. The absence of sufficient fluid motion in the slurry has been identified as a significant contributor to the formation of abrasive particle agglomerations, since the abrasive particles tend to settle out of the suspension unless the slurry is subjected to some fluid movement.
To minimize the possibility of wafer surface scratching due to the presence of tails or agglomerated particles, prior art polishing slurry distribution systems have used fluid filters to trap particles of unacceptable size before the slurry is deposited on the polishing pad of the planarization machine. FIG. 1 shows a typical polishing slurry distribution system according to the prior art. As shown therein, the polishing slurry 12 is retained within a storage tank 11. The slurry 12 is then pumped from the storage tank 11 by a pump 14, and delivered to the fluid filter 16, where the polishing slurry 12 passes through a filter element 17 in a flow direction substantially perpendicular to the surface of filter element 17. After flowing through the filter element 17, the slurry 12 emerges as the filtered slurry 13 that is devoid of either tails or agglomerated abrasive material. The filtered slurry 13 may then be supplied to the wafer planarization machine 18 to be consumed during the wafer planarization process.
A principal shortcoming inherent in the prior art slurry distribution system 10 resides in the fluid filter 16. As shown in FIG. 1, the filter element 17 is generally comprised of a fine interwoven network of polypropylene fibers with open flow areas between the fibers to allow the passage of particles up to a prescribed particle size, and to retain those of larger size. As trapped particles steadily occlude the open areas in the filter element 17, it becomes increasingly restrictive to the flow of slurry, thus limiting the flow of slurry to the planarization pad. Because the flow of slurry continuously deteriorates as slurry flows through the filter 16, continual readjustments to the wafer planarization procedure must be made in order to achieve consistent planarization results. When the flow of slurry is restricted to minimally sufficient levels, the fluid filter 16 (or more generally, the filter element 17) must be removed and replaced. As a consequence, frequent replacement of the filter element 17 commonly occurs in order to achieve a reasonably uniform flow of slurry to the pad over successive wafer planarizations, and to avoid the occurrence of an insufficient slurry flow during any wafer planarization process.
Accordingly, the frequent removal and replacement of the fluid filter 16 makes the wafer more expensive to produce, due to the occurrence of equipment downtime required for servicing the filter, in addition to the cost of the replacement filters.
The problems associated with particle occlusion of the fluid filter 16, as described above, are further exacerbated when slurry compositions are used which have large numbers of tail particles. As briefly described above, slurry compositions that contain a greater number of tail particles are generally less expensive to manufacture. Consequently, an economic incentive exists to utilize these compositions in wafer planarization. Prior art slurry distribution systems, however, have not fully permitted the use of these lower cost slurry formulations, since the useful life of the fluid filter would be substantially shortened. Accordingly, the full economic benefit to be derived from the use of these slurry formulations in wafer planarization has not been realized.
The present invention is directed toward an apparatus and method for the cross flow filtration of polishing slurry compositions used in semiconductor wafer planarization. An apparatus according to one aspect of the invention includes an elongated cylindrical filter element adapted to be rotated at predetermined angular velocities that is disposed within a housing. The housing has an inlet that is fluidly connected to a source of polishing slurry through a pump, an outlet to provide filtered slurry to a planarization machine, and a bypass outlet that is fluidly connected to the source of polishing slurry to allow refiltration of the bypass fluid. A motor is also included to impart rotational motion to the cylindrical filter element. By rotating the filter element in the housing while slurry is flowing through the housing, a fluid shear layer develops on the filter surface that repels larger particles suspended in the slurry from the filter surface, while admitting those of acceptable size to generate a filtered slurry. A portion of the slurry not subject to filtration is routed to the bypass outlet for refiltration.
In an alternate aspect of the invention, the apparatus includes an inner cylindrical filter element and an outer cylindrical filter element that are concentrically disposed within a housing. The filter elements may be independently rotated to achieve multistage filtration of the slurry.